Despite describing the salary of Mark Sullivan as ''obscene'', the Greens sole MLA, Shane Rattenbury, joined with his Labor government partners on Wednesday to defeat an opposition move to ask Auditor-General Maxine Cooper to audit the processes surrounding the ACTEW managing director's wages.

Opposition Leader Jeremy Hanson reacted angrily to the defeat of his private member's motion, accusing the Greens minister of lacking ''integrity'' and ''credibility'', and asking the government what it had to hide.

The opposition's motion demanded a full investigation of Canberra's water utility that probed the $855,000 salary of managing director Mark Sullivan and the $409 million cost of the Cotter Dam.

Mr Hanson also called for a review of the water utility's sponsorship programs and its 50 per cent stake in electricity retailer ActewAGL.

But the government moved amendments that blocked the Liberals' proposal and continued its own inquiry into ACTEW, including how the water utility under-reported Mr Sullivan's salary by $234,000.

Treasurer Andrew Barr said on Wednesday it was not necessary ''to jump to that step'' of a performance audit when several investigations into ACTEW were already under way that covered questions raised by the opposition. Mr Barr said the lines of inquiry included next Monday's special meeting of shareholders with ACTEW's board and the Independent Community and Regulatory Commission's recent report on water prices that raised the issue of governance in ACTEW.

The Treasurer said the Auditor-General had also already proposed an investigation of the governance and administrative arrangements for the review of ACT water and sewerage prices.

''There are a number of different steps in this process and they need to be gone through one step at a time,'' he said.

The government's amendments promise a report back to the Assembly on all issues by June and a report on Monday's extraordinary meeting at the next sitting in May.

Mr Rattenbury told the Assembly he supported the amendments because they promised ''a clear path for what is happening and is going to happen in relation to ACTEW and keeps the Assembly informed of that process''.

But Mr Hanson said the government was trying to distance itself from its role in the controversy. He accused Chief Minister Katy Gallagher of trying to ''absolve herself of any responsibility''. ''It's not surprising the government wouldn't want any light shone on this matter beyond what has already been shone, because they have got some skin in the game as shareholders, because they haven't been doing their job and they don't want that exposed,'' he said. ''But I think that the real loser in this, anyway, is Shane Rattenbury.''

The Opposition Leader accused the Greens minister of ''doing the government's bidding'' despite making public statements that Mr Sullivan's salary was ''obscene''.

''Where is his integrity?'' Mr Hanson said. ''It's remarkably absent. He's got very little credibility left.'' However, Mr Rattenbury said he was satisfied the process the government had put in place ''will get to the bottom of the issues that have been raised''. ''To simply describe taking one approach or the other as some sort of cover-up or not allowing full scrutiny really is simply a political stance by Mr Hanson,'' he said.

''In the amended motion, we will have a series of reports that will be made publicly available, and a date on which they will be publicly available is specified.

''It's certainly far more concrete than saying we want some broad-ranging inquiry which could take a significant amount of time.''